Methyl bromide (bromomethane, MeBr, CH3Br) is used as a fumigant to control pests and to protect agricultural commodities, among other things. For example, methyl bromide may be used as a fumigant on many products, including fruits, nuts, vegetables, plants, bulbs, seeds, cotton, tobacco, bailed hay, lumber, railroad cars, ships, and garbage. Soil or field (preplant) and quarantine (post-harvest) fumigations are the main sources of atmospheric methyl bromide emissions. Emissions from quarantine fumigations are typically highly discontinuous with a relatively small air stream containing an initially high, but exponentially declining methyl bromide concentration.
Unfortunately, methyl bromide has been shown to have a significant ozone depleting potential, being responsible for approximately 5-10% of the global ozone destruction, as well as being acutely toxic. Current fumigation procedures with methyl bromide do not prevent the escape of the methyl bromide. Food and structural fumigations directly emit methyl bromide into the atmosphere without any system of recovery and/or destruction of methyl bromide. In preplant or soil fumigations, greater than 80% of the applied methyl bromide dose escapes to the atmosphere (Yagi et al., 1993, Agricultural soil fumigation as a source of atmospheric methyl bromide, Proc. Natl. Acad. Sci. 90: 8420-8423), the rest being absorbed or degraded by the soil (Shorter et al., 1995, Rapid degradation of atmospheric methyl bromide in soils, Nature, 377:717-719; Serça et al., 1998, Methyl bromide deposition to soils, Atmosph. Environ., 32:1581-1586). The use of polyethylene (PE) tarps covering the soil during fumigation has little effect on controlling emissions because of the high diffusivity of methyl bromide through PE and its slow biotic and abiotic degradation rates in soils (Gan and Yates, 1998, Recapturing and decomposing methyl bromide in fumigation effluents, J. Hazardous Materials, 57:249-258; Yagi et al. supra).
Since fumigant alternatives to methyl bromide are still lacking, several techniques have been proposed over the past few years to treat or recycle methyl bromide contaminated air. These techniques include adsorption, chemical reaction in an aqueous phase, and thermal decomposition in a burner (Wontner-Smith et al., 1998, Reduction of emissions of methyl bromide from chamber fumigation, International Pest Control, January/February 1998, 14-18; Gan and Yates, supra; Nagji 1996, Recycling and recovery of methyl bromide fumigant, U.S. Pat. No. 5,505,908; U.S.D.A., 1996, Recapturing methyl bromide emissions, Agricultural Research Service/Vol. 2, No. 2/April 1996, Web site http://www.ars.usda.gov/is/np/mba/april96/leesch.htm). While adsorption either on activated carbon or on zeolites has shown some promise (Wontner-Smith et al., 1998, supra; Nagji 1996, supra), this process poses hazards as a result of the spent adsorbent, the high concentrations of methyl bromide sorbed involved, and uncontrolled release of the sorbed methyl bromide (Gan and Yates, 1998, supra). Also, it was noted that the adsorption capacity of activated carbons is often insufficient for adsorbing the high loads of methyl bromide vented from chamber fumigations resulting in only partial capture of methyl bromide (Wontner-Smith et al., 1998, supra). A major challenge for treatment of methyl bromide fumigations appears to be the relatively wide range of methyl bromide concentrations to be treated (0.02-45 g/m3) and the discontinuous flow of the exhaust.
Thus, there remains a need for a system and method for treating methyl bromide emissions that is economical and environmentally friendly among other things.